Linear positional precision over 60 feet of movement

snowman · May 15, 2016

Ok, precision is relative I guess. I need to build a device that can have planned linear movement (on a track) across 60 feet. True stopping position must be within 1/16 of programmed stopping point.

Speed isn't really of great concern.

Gear Racks, Chain, belt, etc. I know that it can be done with the double helical, precision rack....but I'm not sure how to spec out precision class, or even if that is really going to be of great concern as long as circular pitch is maintained between the gear teeth at rack joints.

If I'm just using a run of the mill gear rack from McMaster carr, how to I determine total possible error over 60 feet of movement? (based upon perfect theoretical alignment of pitch diameters / circular pitch of rack / etc...just the compounded tooth error)

OR

Is there a simple way of measuring linear position. Similar to a glass encoder, but just really long.

alexdobbie · May 15, 2016

Can you bounce a laser off a paddle on the moving object ?

DMF_TomB · May 15, 2016

snowman said:
Ok, precision is relative I guess. I need to build a device that can have planned linear movement (on a track) across 60 feet. True stopping position must be within 1/16 of programmed stopping point.

Speed isn't really of great concern.

Gear Racks, Chain, belt, etc. I know that it can be done with the double helical, precision rack....but I'm not sure how to spec out precision class, or even if that is really going to be of great concern as long as circular pitch is maintained between the gear teeth at rack joints.

If I'm just using a run of the mill gear rack from McMaster carr, how to I determine total possible error over 60 feet of movement? (based upon perfect theoretical alignment of pitch diameters / circular pitch of rack / etc...just the compounded tooth error)

.
from string pots to magnetic tape reader to laser interferometer to read distances
.
rails even when set straight can bend curved or wavy on temperature changes. optical instruments can see this. important to account for length differences as temperature changes

snowman · May 15, 2016

Let me set this more straight...the traditional way of doing this is a 100 foot tape and soap marks. Works fine. Likely not actually holding 1/16, but close enough. So, I'm shooting for 1/16 as a goal. And yes, temperature changes, all that fun stuff...I know it comes in to account.

Laser is an option, the precision is there, just not inexpensively that I have found as of yet. Most simple time of flight laser distance measurement sensors are +/- 5mm

The mechanical options, I'm less keen on. Sag on the string pots, temp differential, all that starts to come in to play. It would be easy enough to have a quick and dirty calibration on a daily basis, but it makes it a little more confusing.

Heavey Metal · May 15, 2016

DLR13K | Digital Laser Distance Measurer | Bosch Power Tools

snowman · May 15, 2016

Yup...I have one of those. Good for quoting purposes.

If I could get a serial output of the LCD screen, I could probably work with it.

snowman · May 15, 2016

Inexpensive is a couple hundred bucks. Or availability on used market.

I have an RFQ in with micro epsilon to see what the rugged industrial version costs...but it seems that none of the time of flight sensors are really showing up on the used market as of yet. Nor does it seem that anyone has hacked the bosch units...which sort of surprises me as well.

I don't mind paying a couple grand once I have a working prototype. I just don't want to blow that kind of cash on a toy.

Automation direct stuff won't work for me. Analog output...don't want to deal with that. I'd rather use machine vision to read a tape measure!

Monarchist · May 15, 2016

snowman said:
Let me set this more straight...the traditional way of doing this is a 100 foot tape and soap marks. Works fine. Likely not actually holding 1/16, but close enough. So, I'm shooting for 1/16 as a goal. And yes, temperature changes, all that fun stuff...I know it comes in to account.

Laser is an option, the precision is there, just not inexpensively that I have found as of yet. Most simple time of flight laser distance measurement sensors are +/- 5mm

The mechanical options, I'm less keen on. Sag on the string pots, temp differential, all that starts to come in to play. It would be easy enough to have a quick and dirty calibration on a daily basis, but it makes it a little more confusing.

Classic closed-loop / open loop issue.

Build a mechanism so stout, so resistant to change, so difficult to damage or tolerant thereof, so cleverly compensated, that it can be trusted to be where it is supposed to be, and for scores of years.

That became too heavy, too expensive, too costly, and just not GOOD enough - for an increasing percentage of... most things mankind needs.

So we take it as it is, and measure where it is instead.

Laser is the fair-haired not-so-new-anymore kid for that. Beam can be blocked under some circumstance, of course. But that can itself be treated as an 'alarm' condition - even safety. And it recovers cheaply with low/no maintenance cost.

Simply attaching a good quality graduated tape to a rail where it won't be TOO damned torn-up to require overly frequent replacement is older. Not that expensive when you need but the sixteenth part on an INCH. CAN be good enough to have been built-in to a Cadillac Gage 'Pla Check' of days gone by.

OTOH .. those weren't meant to go play in the dirt. Your kit will probably have to do, and for long years.

Durability. low-maintenance, even 'future proofing' suggests laser.

If it 'appears to' cost more up-front, its eventual replacements will become progressively cheaper, even if not also more accurate. 'Invar' tapes and their tensioners - other traditional counterparts, even down to the common carpenter's tape measure - have done the reverse. Gone scarcer and more costly, if even easily still found at all.

Bill

Heavey Metal · May 15, 2016

Bosch GLM 1 C Professional – Direct digital transfer of measuring results

Blue tooth

Or micro usb

Monarchist · May 15, 2016

Heavey Metal said:
Bosch GLM 1 C Professional – Direct digital transfer of measuring results

Blue tooth

Or micro usb

"Millimeter" precision? That's what? About a 25th of an inch this side of the water?

Less than a sixteenth, anyway...

CalG · May 15, 2016

Specs read +/- 1.5mm that's good stuff over 20 meters.

Monarchist · May 15, 2016

CalG said:
Specs read +/- 1.5mm that's good stuff over 20 meters.

+/- about 60 thou izzat?

Hard to do better optically over any short-enough period of observation/averaging....unless you can evacuate all the pesky AIR in the room as well as controlling temp and particulates.

That does, however, raise the question. Where does the equipment have to operate, environment-wise?

Extensible boom on a dredger in muddy water, or..

Bill

imported_brian_m · May 15, 2016

We use a custom laser setup for doing what you are considering but our focus is on speed of operation versus absolute accuracy.
You will find that a laser system is a trade-off, accuracy versus speed of operation versus the maximum operating range.
I have the specs for the laser we use right next to me. Absolute accuracy is plus/minus 25 mm. with a repeatably accuracy of less than 5mm. This laser can read up to 50 meters. With the reduced range models the accuracy increases. You should be able to get down to the accuracy you need but it depends very much on your actual application.
This laser is a high quality European product so I doubt that other available units will show much improvement in performance. Cost around $500 to $600 just for the laser sensor head, then you need to add the hardware and software to adapt it to your use.
Having said that I think a laser is the way to go because the various mechanical systems we considered, and some we actual used, all have long term maintainability issues.

Monarchist · May 15, 2016

imported_brian_m said:
the way to go because the various mechanical systems we considered, and some we actual used, all have ^^^ worse ^^^ long term maintainability issues.

Fixed that for yah.

"Bubba' can always find a way to smack $600 worth of sensor head to flinders, even on Day One. They just hide from his notice better than some other gadgetry.

Or he gets tired of trying to steal the beam, stuff it into a Ziploc bag, and take thutty-foot worth of it home for the kids to play with.. figuring no one will miss it..

Bill

Heavey Metal · May 15, 2016

CalG said:
Specs read +/- 1.5mm that's good stuff over 20 meters.

100 meters

Monarchist · May 15, 2016

Heavey Metal said:
100 meters

Aye, and bring enough money another full order of magnitude is on the shelf if one needs that.

Just the other day, there was a MAJOR improvement published in an altered methodology for more accurately measuring the diameter of Black Holes. The ones in outer space, not the ones your favorite micrometer vanished into. Great deal more than 100 meters away, those are.

Bill

wheels17 · May 15, 2016

I'd also look at the Leica Disto products. I have one that seems to be very accurate, although I don't have any quantitative accuracy data.

They have a number of data output options on various devices, including bluetooth and wi-fi.

Same +/- 1 or +/-1.5 mm "typical measuring accuracy".

http://www.gpspartners.lv/sites/gpspartners/files/media/files/leica_disto_series_bro_en.pdf

imported_brian_m · May 15, 2016

The specification of "absolute accuracy" versus "repeat accuracy" is important.
"Absolute accuracy" applies when you want to randomly measure any position on the track. "Repeat accuracy" applies when you are sensing the position of fixed points on the track such as positions A ,B or C.
On all laser specs. I have seen repeat accuracy is, as you would expect, much better than absolute accuracy so if you want to send a tool accurately to a random position this is going to be much more difficult than sending the tool accurately to a "pre-surveyed" position.
Conceivably, if you had a permanent track, you could calibrate it so that the software applied a correction to the randomly measured position depending on the tools position on the track so increasing random measurement accuracy. Just a crazy idea but it would add accuracy to your positioning if you needed a further aid..

snowman · May 15, 2016

imported_brian_m said:
The specification of "absolute accuracy" versus "repeat accuracy" is important.
"Absolute accuracy" applies when you want to randomly measure any position on the track. "Repeat accuracy" applies when you are sensing the position of fixed points on the track such as positions A ,B or C.
On all laser specs. I have seen repeat accuracy is, as you would expect, much better than absolute accuracy so if you want to send a tool accurately to a random position this is going to be much more difficult than sending the tool accurately to a "pre-surveyed" position.
Conceivably, if you had a permanent track, you could calibrate it so that the software applied a correction to the randomly measured position depending on the tools position on the track so increasing random measurement accuracy. Just a crazy idea but it would add accuracy to your positioning if you needed a further aid..

But why is that? I have seen the same....linearity is off, but repeat accuracy is on. I guess it's just because you are just looking for the exact same output?

freeflyer · May 15, 2016

If this is a repeat move to the same position why not just make a stop with a precision limit switch? Do the initial calibration, then it will repeat.
Michael

Linear positional precision over 60 feet of movement

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